Regular inspection and the correct maintenance of generators and their associated control gear is essential to prevent failure and inefficient operation.
Always ensure that the generator prime-mover is shut down and locked off before you begin any maintenance.
Also ensure that the generator circuit breaker is locked off, auto-start circuits are disabled and electric heaters are switched off and isolated.
All wiring to the generator should be inspected for damage or frayed insulation and tightness of terminal connections.
Particularly check for signs of oil and water contamination of cable insulation within terminal boxes.
Check that the cooling air intake and exhaust openings are not blocked and are free of dirt and dust.
Inspect and clean the ship generator rotor and stator windings by removing dust with a dry lint-free cloth.
Low pressure, dry compressed air may be used to dislodge heavier dirt but be careful not to drive the dirt deeper into the windings. An industrial type vacuum cleaner is very effective for removing dirt from the windings.
Use a rubber or plastic coated nozzle on the vacuum cleaner tube to prevent abrasive damage to the sensitive winding insulation.
Oil on the surface of winding insulation will reduce the insulation resistance and shorten its life.
The oily deposits can be removed by washing the windings with special degreasant liquids. Minor abrasions to winding insulation can be repaired, after cleaning, by the application of a suitable air-drying varnish.
Rotor slip rings must be checked for uniform (even) wear and that the carbon brushes have free movement in their boxes. Correct brush pressure can be checked using a pull-type spring balance and compared with the manufacturer’s instructions.
A pull of around 1-1.5 kg is usual.
If the brushes on ship generator become too short (below about 2 cm) the reduced spring pressure will cause sparking at the slip ring contact.
Replace brushes with the correct type and bed them to the curvature of the slip rings. This can be done by placing a thin strip of glass paper (not emery paper) over the slip ring with its cutting surface under the carbon brush. Pull the glass paper around the slip ring until the brush surface has the same contour as the ring.
The last few passes of the glass paper should be made in the same direction as the normal rotor direction.
Remove all traces of carbon dust with a vacuum cleaner.
Generator excitation transformers, AVR components and rotating diodes must be kept free of dirt, oil and dampness.
A special contact grease is used between the diode connections to prevent electrolytic action occurring between dissimilar metals.
Check such contacts for tightness but do not disturb them unnecessarily.
Measure the insulation of the stator and rotor windings to earth and between stator phases (assuming that the neutral point is available for disconnection at the terminal box).
Remember to disconnect or short-circuit any electronic circuit components which are likely to be damaged by u high voltage insulation test.
Consult the wiring diagrams and the manufacturer’s instructions before testing.
Record the IR values and note the prevailing temperature and humidity.
Compare with previous test results.
A minimum IR value is usually taken to be 1 MOhm but a lower value may be acceptable to a surveyor based on 1 kOhm / volt.
However, it is the historical trend of the machine IR values which will give a better picture of the insulation condition.
Generators with very low IR values (less than 0.5 MOhm) should be given a thorough cleaning then dried out.
If the IR has recovered to a reasonable value which has become steady during the drying period, its windings should be covered with high-quality air-drying insulating varnish.
Should the IR value remain low during a dry-out, the machine insulation needs to be completely reimpregnated or rewound (generally by a specialist contractor).
After maintenance, no-load running checks should precede synchronising and loading.
On load, particularly check for excess temperature rise and loadsharing stability when running in parallel.
Finally, if a generator is to be left idle or a long time, make sure that its windings are suitably heated to prevent internal condensation forming on its insulation.
As with all electrical equipment – dirt, overheating and dampness are the enemy!